Recently, an internal combustion engine which can use many kinds of fuel (petroleum fuel, alcohol fuel, mixed fuel of petroleum fuel and alcohol fuel) has been developed. A stoichiometric air-fuel ratio of the fuel increases with the alcohol concentration. A fuel quantity supplied to the engine is controlled so that the air-fuel ratio agrees with the stoichiometric air-fuel ratio. The fuel quantity supplied to the engine varies according to the alcohol concentration.
A fuel supply system for supplying fuel in a fuel tank into the engine is provided with a pump device pumping the fuel in the fuel tank and an injector injecting the pumped fuel into the engine. In the fuel supply system, when the pump device has a malfunction, fuel pressure of the fuel supplied to the injector is decreased so that the fuel supply to the engine becomes insufficient. JP-11-190240A (U.S. Pat. No. 6,125,832) shows a fuel supply apparatus in which fuel-supply -pressure to an injector is detected and a malfunction of a pump device is detected based on the detected fuel-supply-pressure. However, if the apparatus shown in JP-11-190240A is applied to the engine which can use many kinds of fuel, following problems will occur. That is, when the fuel supply quantity to the injector is increased according to the alcohol concentration of the fuel, a fuel-supply-pressure drop due to the fuel injection is increased. As the result, even though the pump device has no malfunction, it is determined that the pump device has malfunction. An accuracy of malfunction detection of the pump device is deteriorated.